1. Field of the Invention
The present invention relates mainly to a power amplifier in a portable telephone or the like for mobile communication.
2. Background Art
Radiofrequency power amplifiers for portable telephones in code division multiple access (CDMA) schemes or the like include ones using a complementary metal oxide semiconductor (CMOS) cascode amplifier for a reduction in manufacturing cost. The development of such power amplifiers is now being promoted. Variation in gain of a cascode amplifier with respect to input power is an AM-AM characteristic, while variation in phase is an AM-PM characteristic. There is a correlation between the AM-AM characteristic and a distortion characteristic. If the amount of AM-AM change can be reduced, the distortion characteristic can be improved.
A technique for adjusting a gain frequency characteristic S21 at 0 to 60 GHz (a frequency dependence of the gain with respect to constant input power) of a cascode amplifier, particularly the amount of leap in gain has been proposed (see, for example, Japanese Patent Laid-Open No. 2003-92523). This technique, however, is not a technique for adjusting an AM-AM characteristic (or an AM-PM characteristic) of a cascode amplifier.